Designing and Building Hurricane-Resistant Homes

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					Designing and Building Hurricane-
Resistant Homes
Mercedes Homes and its partners set out to create and
construct a truly hurricane-resistant house.

by William Zoeller

       production builder’s efforts to
       identify better wall systems to use
       in homes led to the development
of a disaster-resistant housing solution for
the southeastern United States—a solu-
tion that proved its worth last October
when Hurricane Wilma slammed into
that region of Florida. Florida-based
home builder Mercedes Homes started
working with DOE’s Building America
research teams in 2000 to develop an
advanced wall system to replace the con-
crete masonry unit (CMU) block con-
struction typically used in Florida.After
researching and prototyping a variety of
systems, including precast concrete, Mer-

                                                                                                                                                       WILLIAM ZOELLER
cedes Homes selected a cast-in-place
concrete wall system.
    Beginning in 2001, the concrete wall
system, dubbed the Solid-Wall-System
(SWS) by the builder, was rigorously
examined and engineered by researchers         Mercedes Homes’ concrete wall system starts with a steel-reinforcing cage consisting of a 6 x 6 steel
from the University of Florida’s Pro-          mesh and vertical #5 steel bars placed at 48 inches on-center.
gram for Resource Efficient Commu-
nities and Steven Winter Associates,
Incorporated (SWA), using funding              tropical storms pounded Florida and the              season, Mercedes Homes and its partners
provided by the Federal Emergency              Southeast. In evaluating the aftermath,              set out to design and build a truly hurri-
Management Agency (FEMA). The                  SWA and University of Florida                        cane-resistant house.The prototype was
results of those studies were impressive,      researchers found that the concrete                  completed in early 2005 at Huntington
showing a marked potential perfor-             homes performed exceptionally well in                Lakes, a Mercedes Homes Community in
mance improvement during windstorm             resisting hurricane force winds, includ-             Rockledge, Florida.This house combines
events, as compared to the regionally          ing hurricane-induced uplift and lateral             a number of advanced building technolo-
standard CMU walls. In 2001 Mercedes           loads; these walls even helped protect               gies to offer homeowners three levels of
Homes started implementing the SWS             the homes from wind-blown debris.                    hurricane protection: superior structural
as a standard offering. By the spring of       What became evident, however, was                    strength; greater resistance to wind-driven
2004, Mercedes had built cast-in-place         that most of the damage to buildings                 rain; and improved poststorm recovery. In
concrete walls on more than 2,000              throughout the region resulted not from              addition to hurricane “hardness,” the pro-
Florida homes; by October 2005, that           catastrophic structural failures, but from           totype has several features that increase
number had grown to nearly 4,000.              wind-driven rain intrusion.                          energy efficiency.These include insulated
    The 2004 hurricane season provided             Combining the original structurally              low-e windows; an air handler located in
an extensive real-world test for the con-      based engineering research with the                  conditioned space (now a standard Mer-
crete houses when a record number of           lessons learned from the 2004 hurricane              cedes practice with its new plans); a 13-

4                                                                         HURRICANE SEASON 2006 • HOME ENERGY
                                                                                                                        posite system with superior resistance to
                                                                                                                        hurricane-induced forces.Typical wind
                                                                                                                        speeds for a Category Five tropical storm
                                                                                                                        range up to 155 mph, with wind-borne
                                                                                                                        debris traveling well upward of 100
                                                                                                                        mph. Engineering analysis and destruc-
                                                                                                                        tive testing show that the cast-in-place
                                                                                                                        system can withstand missiles fired at up
                                                                                                                        to 200 mph.
                                                                                                                            Structural strength is not the only
                                                                                                                        advantage of the cast-in-place system.
                                                                                                                        Concrete walls have high thermal mass,
                                                                                                                        reducing energy transfer between inte-
                                                                                                                        rior and exterior surfaces.This in turn
                                                                                                                        reduces the homeowner’s energy bills.

                                                                                                      WILLIAM ZOELLER
                                                                                                                        The continuously poured system mini-
                                                                                                                        mizes interior temperature fluctuations
                                                                                                                        and drafts and reduces noise transmis-
                                                                                                                        sion. Rigid foam insulation is laminated
                                                                                                                        directly to the interior surface of the
                                                                                                                        concrete before the roof trusses and
 More than 4,000 concrete SWS homes have been built, including this community in Rockledge,                             interior partitions are installed.This sub-
 Florida.                                                                                                               stantially simplifies the construction
                                                                                                                        process and ensures a continuous insu-
                                                                  in much of Florida consists of                        lation layer with no breaks or gaps.
                                                                  CMU walls for single-story                                Another advantage of concrete is that
                                                                  homes.Although CMU con-                               it inherently resists mold, termites, and
                                                                  struction is stronger and more                        rot, because it contains no organic matter.
                                                                  durable than standard frame                           Concrete may also be inherently water-
                                                                  construction, CMU-con-                                proof, depending on its ultimate strength;
                                                                  structed homes, when sub-                             5,000 psi is virtually waterproof, while
                                                                  jected to a tropical storm, are                       3,500 psi is water resistant. For these
                                                                  still susceptible to water intru-                     homes, the exterior finish coatings in
                                                                  sion, lateral and uplift wind                         combination with the 3,500 psi concrete
                                                            WILLIAM ZOELLER

                                                                  load failure, and penetration                         provide complete waterproofing. The
                                                                  damage caused by wind-dri-                            moisture that can enter CMU walls at
                                                                  ven debris (“missiles”).                              joints, especially when the joints are
                                                                       The SWS homes are con-                           deformed by structural load, is eliminated
                                                                  structed using 3,500 psi con-                         in the monolithic system, as is the poten-
The structural superiority of the new Mercedes models
                                                                 crete placed in modular                                tial for water storage within the hollow
comes primarily from the cast-in-place concrete wall system      aluminum forms to create the                           CMU cavities. And the houses are safer
—the SWS.                                                        6-inch walls. The concrete                             from fire as well as water; with their 6-
                                                                 encloses a steel reinforcing                           inch solid concrete walls, they have up to
                                                                 cage of 6 inch x 6 inch road-                          a three-hour Class A fire rating.
SEER heat pump; and Energy Star                       mesh combined with vertical reinforcing
appliances.This combination of features               bars placed at 4-ft intervals.The cage is                         Wind-Driven Rain
proves that builders can construct cost-              specifically designed to evenly distribute                         Protection
effective, energy-efficient, hurricane-resis-          direct wind, lateral, and point loading
tant homes—providing true value and                   from missile impact across the wall sur-                             Wind-driven rain can enter the
security to their customers.                          faces to minimize point stresses, which                           home at vulnerable points from foun-
                                                      can lead to failure.The cage connects to                          dation to roof. Water intrusion at
Structural Protection                                 steel in the concrete slab, and the roof                          ground level can be mitigated by creat-
                                                      trusses are held by wet-set tie-downs (or                         ing shallow indentations or recessed
    The structural superiority of the new             hurricane straps) embedded every two                              seats in the foundation slab to prevent
Mercedes models comes primarily from                  feet in the concrete as the walls are                             rain from being driven or sucked into
the cast-in-place concrete wall system—               poured. The steel-reinforced concrete                             the home under the exterior doors and
the SWS.Typical residential construction              wall assembly acts as a monolithic com-                           walls. The recessed seats work well in

HOME ENERGY • HURRICANE SEASON 2006                                                                                                  5
conjunction with out-swing entry
doors—a somewhat unconventional but
practical solution to a common source
of damage. In the aftermath of the 2004
storm season, researchers found that
high wind forces on in-swing entry
doors allowed considerable water to
enter the home, because the weather-
stripping gaskets rely on pressure in the
opposite direction. Complete blow-in
failure of an in-swing door during hur-
ricane-force winds can result in signifi-
cant water damage, and even to changes
in internal pressure that contribute to

                                                                                                                                                WILLIAM ZOELLER
roof uplift and significant structural
damage. Following Hurricane Wilma in
October 2005, Mercedes found that
owners who had opted for in-swing
doors, rather than the now-standard
out-swing configuration, suffered more
instances of damage. As a final protec-       SWA and Mercedes continue to collaborate on hurricane-resistant homes, including this two-story
                                             concrete prototype currently under construction.
tion for openings, the builder offers
removable hurricane shutters as an
option to all home buyers.
    Exterior wall surfaces are protected
with elastomeric sealant at form joints
and at the concrete form snap-tie loca-
tions.Although concrete is poured con-
tinuously across multiple forms, the
change in texture at vertical form joints
and tie connections creates a vulnerabil-
ity to water infiltration.Applying sealant
to these form joints is a simple and
effective way to prevent damage.A con-
tinuous drainage plane covers roof
trusses on gable ends to prevent sheet-
ing water from entering the building
assembly at the truss-to-wall transition.

                                                                                                                                               WILLIAM ZOELLER
In addition to the house wrap that is
typically installed over the vertical wall
sheathing, a separate building-paper-
backed wire mesh is installed over the
house wrap before the stucco finish is
applied. This creates a foolproof
drainage plane and makes for proper          The recessed seat at the edge of the floor slab makes it more difficult for storm-driven water to
                                             penetrate the joint between the concrete walls and the floor slab.
stucco adhesion. Wall surfaces are fin-
ished with a high-performance acrylic
coating, which is capable of flexure and      of lost or damaged shingles—a common                 face openings limits water intrusion
which bridges small gaps, to prevent the     casualty of tropical storms. And the                 while encouraging greater air circula-
exterior walls from absorbing water          envelope often fails when water sheeting             tion—and faster drying—within the
during sustained heavy rainstorms.           off the roof spills down the fascia board            eave assembly.A simple change to baffled
    At the roof level, researchers recom-    and is driven by wind or surface tension             ridge and roof vents was also used to
mended several strategies to protect         into the soffit vent openings.To prevent              prevent water form entering the struc-
against water intrusion.A peel-and-stick     this from happening, a redesigned fascia             ture. Typical roof vents are not config-
underlayment was adhered directly to         extends 1 inch below the soffit to form               ured to prevent the entry of horizontal
the roof decking beneath the shingles to     a drip edge, directing water down and                rain, while the baffled designs do just
create a secondary roof drainage plane.      away. A new perforated soffit board                   that (see Figures 1 and 2).
This provides backup protection in case      product with recessed rather than sur-

6                                                                       HURRICANE SEASON 2006 • HOME ENERGY
Poststorm                                                                                                                   SWS is incrementally more
Recovery                                                                                                                    expensive than the CMU
                                      Typical Soffit Design                                                                  construction it replaces, the
    Most of the losses from the                                                                                             benefits to the homeowners
2004 hurricane season resulted                                                                                              and to the builder—produc-
from water intrusion.When rain                                                                                              tion time for the walls is
or moisture wets parts of the                                                                                               reduced from five days to
wall or floor assembly, mold can                                                                                             two—easily outweigh the
grow indoors undetected for                                                                                                 costs.The now-standard water
many months, compromising                                                                                                   intrusion and storm recovery
indoor air quality and causing                                                                                              measures add about $2,200 to

                                                                                                         WILLIAM ZOELLER
significant long-term damage to                                                                                              the cost of the homes, and the
the home. Water damage and                                                                                                  upgrade options (generator
mold growth become more                                                                                                     and shutters) add another
severe when power outages pre-                                                                                              $8,000.
vent the homeowner from dry-                                                                                                    By the time Hurricane
ing out the home quickly                                                                                                     Wilma scored a near-direct
following a storm.                 Figure 1. The typical soffit design for new home construction in                           hit on the Melbourne area in
    To aid in poststorm recovery, Florida is susceptible to water intrusion from direct wind pressure,                       October, Mercedes had com-
a natural gas-fueled generator is suction on the lee-ward side of the house, and from surface tension as                     pleted and sold 343 new
offered as an option to home the water sheets off the roof and clings to the fascia and soffit.                               homes built using the
buyers throughout the Florida                                                                                                improved specifications. In
market. (The natural gas grid                                                                                                only 16 of these homes did
tends to be far more robust           Improved Eaves Design                                                                  the owners report any dam-
than the electrical grid, as vir-                                                                                            age—and all of the damage
tually all the natural gas infra-                                                                                            was      minor.    Problems
structure is below grade. Even                                                                                               included water intrusion
if electrical power lines are                                                                                                from two in-swing front
below grade, transfer stations                                                                                               doors, which were installed at
and transformers often are                                                                                                   the owner’s request, a ridge
not.) The unit allows home-                                                                                                  vent leak where the old
owners to use shopvacs, fans,                                                                                                design was inadvertently
and dryers during poststorm                                                                                                  used, and shingle damage
recovery when power outages                                                                                                  where no leak was reported.
                                                                                                          WILLIAM ZOELLER

are common, greatly reducing                                                                                                 SWA and Mercedes continue
the risk of mold growth and                                                                                                  to research and develop
long-term damage. Mercedes                                                                                                   advanced hurricane-resistant
Homes also offers a generator-                                                                                               housing solutions including
ready electrical service panel                                                                                               two-story cast-in-place wall
that can be easily connected to Figure 2. The improved eaves design features an extended fascia                             systems, as well as perfor-
a portable generator. Even if the forming a drip-edge to defeat the rain water's surface tension, and a                     mance testing of more con-
homeowner cannot initially reconfigured vented soffit with recessed perforations arranged along                               ventional building assemblies.
afford to buy a natural gas gen- the top of perpendicular ridges. Additional blocking is also installed                     With ongoing improvements,
erator, each home is now to prevent soffit blow-out.                                                                         Mercedes expects to see even
prewired and equipped for a                                                                                                 less damage, and more happy
generator to be installed at a                                                                                              homeowners, in the future.
later date, as needed.                        tional paper-faced products. Removing
    To help homeowners to remove              damaged drywall, and the mold it sup-                 William Zoeller is an architect and senior
moisture if water entry occurs, the           ports, was one of the most common                     associate with Steven Winter Associates,
houses are built with nonorganic finish       storm-related repairs faced by home-                  Incorporated, of Norwalk, Connecticut.
materials. Mold grows particularly            owners following the ’04 hurricanes.
quickly on paper, adhesives, and other
organic matter. The storm-resistant                                                                     For more information:
                                              Major Improvements,
prototype utilizes a new paperless dry-                                                                 For more information on SWA
wall product offered by Georgia               Minor Damage
                                                                                                        and Mercedes Homes, go to
Pacific.This product uses a glass-rein-          The concrete SWS homes are now                and
forcing mat facing over gypsum; it            standard in several Mercedes Homes              
resists mold growth better than tradi-        divisions in central Florida. Although

HOME ENERGY • HURRICANE SEASON 2006                                                                                          7

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